Description of Prior Art
Radar data is transmitted and received with a horizontal scanning antenna and detected radar returns are time organized along a time/distance radial for each given angle of antenna position. Thus, the conventional term rho-theta, or radial scan, was coined to describe the data pattern of radar returns. In radar infancy, long delay phosphor cathode ray tubes or storage tubes were used to display the radar returns, and data was written on the display in antenna-synchronized radial scan format, resulting in a sector of displayed radar.
Radially scanned sector displays however suffer in pictorial quality because of lower resolution (wide scan line separation) at the periphery of the display sector than at the apex. Thus the conventional radial scan format utilized in radar displays gave way to arc scan, where data is scanned onto the cathode ray tube orthogonally to the radar returns.
Radial scan display format causes the cathode ray tube beam to repeatedly scan from the apex towards the periphery of the display and the display format is of a "spoked" nature. Arc scan format causes the beam to constantly scan successive ones of concentric arcs which define the sector of display. In conventional arc scan systems, the radar data is organized or converted from the .rho./.theta. received format to an arc scan format for successive readout from storage of these information bits for a given .rho. (distance) throughout the scanning of an arc at that distance-defined radial. As such, the arc-scan display is of an improved quality since the data is displayed orthogonally to the radar returns.
While conventional arc scanned displays eliminate the undesirable "spoked" nature of radial-scanned displays, these types of displays currently define a constant scanning time for each arc (utilize a constant horizontal scan frequency). Because of this constant horizontal scan frequency, several display deficiencies remain in conventional arc scan displays. Firstly, since the writing speed at the sector periphery is very fast and at the sector apex is very slow, deflection power remains relatively high to accommodate the fast peripheral scan. Secondly, since the writing speed at the periphery is considerably slower than writing speeds towards the apex on successive arcs in the display, brightness compensation circuits have to be maintained.
The left-over problems introduced by arc scan displays have been subsequently resolved with the introduction of X-Y scanned (TV scan) radar displays. This latter technique, however, introduces a "blocking" or "stair-step" pictorial effect because data is never displayed orthogonally to, or parallel to, radar reception data, except on the imaginary line through the center of the displayed sector. Displays of the TV scan type, while eliminating problems of uneven brightness and relatively high power encountered in arc scan displays, nonetheless provide a display which is of a synthetic or approximated nature due to the stair-step effect.
Accordingly, it is a primary object of the present invention to provide an improved arc scan display system employing a concept of constant writing speed throughout the display, wherein radar data is displayed orthogonally to radar reception data at all points on the display sector.
A further object of the present invention is to provide an improved arc scan cathode ray tube display system wherein peripheral arc scan writing speed may be appreciably reduced from that of conventional arc scan displays, resulting in an appreciably increased display brightness.
A still further object of the present invention is the provision of an arc scan display system employing constant writing speed, wherein the deflection amplifier voltage requirements may be appreciably reduced from those necessary in conventional arc scan display systems.
A still further object of the present invention is the provision of an improved arc scan display system with constant writing speed throughout the displayed sector, wherein the need for brightness compensation is eliminated.
The present invention is featured in means for generating horizontal and vertical deflection signals respectively defined as A sin .omega.t and A cos .omega.t, where A is a predetermined different constant amplitude during each of successive arcs to be scanned and is linearly decreased from a maximum value during scanning of the peripheral one of the arcs to successively lesser values for successive arcs beneath the peripheral arc. The scan rate (.omega.t) is increased from a minimum value during scanning of the peripheral one of said arcs to successively greater values at successive increased rates for successive arcs beneath the peripheral one of the arcs in the display. Means are provided for applying the deflection signals to respective horizontal and vertical beam deflection means of the cathode ray tube such that the writing speed of the beam, in terms of arc deflection increments per unit of time, is a like predetermined constant during scanning of each of the successive ones of the arcs. Means are provided for storing a predetermined like number of display video bits for each of the arcs in the display. As each arc is scanned, the formatted data is displayed. Means are provided for loading the display bits for a given arc into a shift register means prior to the scanning of that arc. A system clock is employed for developing a video clock pulse train having a number of pulses corresponding to the number of display video bits for each arc, with successive ones of the pulses in the video pulse train having a time-position relationship with successive ones of a corresponding number of horizontal deflection linearly incremented beam-defining positions. The shift register means defining the display bits is clocked by this video pulse train, and the bits are displayed in an essentially time-synchronous relationship with respect to the beam position--the synchronous relationship being such that video and beam position are synchronized to within one displayed picture element (video bit) throughout the sector display.